1. Field of the Invention
The present invention relates to apparatus for testing gas sensors and, more particularly, apparatus for testing carbon monoxide gas sensors.
2. Description of the Prior Art
Health and safety hazards that are associated with exposure to carbon monoxide and other gases have led to the development of various types of gas detectors. Such devices are based on various technologies including infrared absorbance, electrochemical oxidation, metal oxide conductivity, and chemical color changes. The different technologies offer various advantages and disadvantages, but each generally requires periodic calibration. Even for those instruments in which calibration is not required, verification of the instrument's functionality is usually preferred. However, such calibration has not always been available.
Various calibration and verification tests have been used in the prior art. The exact mechanism of these tests depends on the sensor technology. Some testers were based on electrical property measurements such as electrical resistance or electrode capacitance as described in U.S. Pat. No. 5,202,637. Other tests are based on measurement of electrochemical properties. However, such prior art testers were subject to various difficulties and disadvantages. For example, such tests tended to provide a simple positive or negative result--if the result is negative the sensor is deemed to be non-functional. However, such prior art tests were designed to assess only selected failure modes. If the sensor were to fail in a non-design mode, the result would be a false-positive indication. For example, an electrochemical sensor would lose sensitivity and yield false-positive results to an electrical resistance or electrode capacitance test if the diffusion limiting membrane were blocked by foreign material. As another example, a number of sensors use catalysts that can be deactivated by poisoning agents. However, electrical tests of the sensor would not detect this fault.
Other sensor testers such as described in U.S. Pat. Nos. 4,151,739 and 4,267,030 relied on an electrochemical electrolysis cell that was incorporated into or near the sensor. The sensor was generally responsive to the gas that the electrolysis cell generated, but the test process was indirect because the generated gas was not the gas of interest. For example, one electrolysis cell described in European Patent Application EP0744620A1 produced hydrogen gas through the electrolysis of sulfuric acid. However, it was recognized that if the gas of interest was other than hydrogen, as, for example, carbon monoxide, it would be preferable to test the sensor by direct exposure to the gas of interest.
A more reliable test methodology is to expose the sensor to the target gas and monitor the sensor response. However, prior art methods generally required the use of heavy or bulky compressed gas cylinders or complex electrochemical reactions to generate the target gas. For example, carbon monoxide sensors in industrial applications are frequently exposed to carbon monoxide sourced from a gas cylinder. Moreover, convenient sources of the target gas are not always available. Thus, in some applications, as in residential usage, the carbon monoxide sensors frequently are never field calibrated at all.
Accordingly, there was a need in the prior art for a gas sensor tester that would expose sensors to the target gas and monitor the reaction of the sensor. Additionally, there was a need for a sensor tester that was relatively compact and light, that would work with various types of sensors, and that provided a reliable, repeatable test environment.